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E39 (1997 - 2003) The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

This is one of the cases where the diagram is misleading. Item #20 is actually on TOP...when you go to the car to look at your set-up...undo the srews on the top of #19...then just screw the bottom section out of the frame. The bottom of #19 screws into the frame that the reservoir is mounted to.

You can see the nut (#20) that is screwed onto the top threaded section of #19 in the pic below. Once you remove those two nuts...the reservoir is loose and can be moved out of the way. You then just unscrew #19 (or what's left of it) out of the frame:

Again, there are no nuts on the bottom of #19...the diagram has #20 (the nut) in the wrong location...as if they screw onto the bottom...when they actually screw on from the top:

I just repaired this same problem yesterday. Hammer and screwdriver. Place the flat screwdriver against the right edge of the metal plate that the rubber has sheared off of such that it will 'bite' into the disc when you rap it firmly with a hammer. The idea is to transfer the force of the hammer into an unscrewing motion. Mine came loose with 5-6 good whacks. Don't be afraid to hit it hard

There's probably no need to whack anything...especially hard. If the rubber thingy (#19) has sheered in half...all you need to do is turn what remains COUNTER CLOCKWISE (you may need a pair of pliers) and the bottom half will SCREW out of the frame (#21). I'd try the "kindler/gentler" approach first...then move up the scale of escalated collateral damage.

The threaded bolt does NOT run all the way through the rubber...so if the rubber has sheered in half...both sections still have a piece of a threaded bolt...the lower section screws into the frame...and the top section is secured with the nut (#20). All you should need is something to grip the remaining lower piece of rubber (like a pair of pliers...or even your fingers (if you have Kung-Fu grip ) and unscrew it from the frame.

See the pics in my post above...you can see that nut (#20) is screwed onto the top threads of (#19)...when you remove the nut....the reservoir can be lifted off the upper part of #19...the rubber mount (#19) can then be unscrewed from the frame (#21)...whether it is whole or been sheered in half.

There are nutplates on the underside of the bracket. Hand-tighten the new rubber mounts into the bracket. No need to hog down on the upper nuts. Note that, after you remove the washers, the cannister will merely rotate away from the bracket.